1. Field of the Invention
This invention relates to the field of underground line location systems and, in particular, to devices for electrically coupling power to concealed objects so that they can be located by an underground line locator.
2. Related Art
Underground line locators are used to locate the position of lines buried in the ground (i.e., underground lines) such as gas pipes, water pipes, telephone cables, and power cables. A line locator system typically includes a transmitter and a receiver. The transmitter can be electrically coupled to the underground line to be located to cause an electric current to flow in that underground line, which results in the emission of a magnetic field from that line. The receiver locates the underground line by detecting the radiated magnetic field from the underground line.
In a direct connection mode, the transmitter is conductively coupled to the line to be located, typically at a point in the line that is above the ground. The transmitter generates a voltage at one end of the line, causing an electrical current to flow along the conductive line, which produces an electromagnetic field around the line. The electromagnetic field penetrates the ground surface and exists above ground where the receiver can detect it.
In instances where direct connection to the line is not possible (e.g., the line is completely underground), the transmitters of line locators can operate in an inductive mode. The transmitter of an inductive mode line locator produces current in the line by mutual electromagnetic induction. A time-varying electromagnetic field is radiated by an antenna and induces a current in the line to be located. The current produces an electromagnetic field around the line that can be detected by the receiver.
To locate the line, an operator typically moves the receiver over the surface of the ground until the receiver indicates the location of the source of the magnetic field and, therefore, the location of the underground line. The ability of the receiver to locate a line depends on the strength of the electromagnetic field, which is proportional to the electrical current in the buried line. According to Ohm's Law, the current is inversely proportional to the impedance of the line. Because of the wide variability of the physical condition of buried lines and the wide variability of the environment in which those lines are buried, the impedance of buried lines can vary over a wide range. The current produced by the transmitter of a typical locator system varies with the different impedance encountered in each line. The strength of the magnetic field generated by the current within the line, then, varies with the impedance and determines whether or not a receiver above the ground can locate the line with any accuracy.
However, it may be necessary to control the output voltage of the transmitter in order to prevent damaging the underground line or to comply with regulations such as those of the Federal Communications Commission. At present, conventional transmitters in line locator systems control the voltage applied to the underground line, and therefore allow the output current to depend on the impedance of the line, while attempting to maximize the electrical power transmitted into the line. However, the resulting magnetic signal generated from the underground line varies with the impedance of the underground line such that the ability of the receiver to locate the line also depends on the impedance of the line, which can vary widely from location to location.
Therefore, there is a need for transmitters in line locator systems which result in magnetic signals which are independent of the impedance of the underground line.